This invention relates to chemical processes in which a liquid is contacted with a counterflow of gas. This may be for a variety of purposes such as stripping a component from the liquid stream or absorbing a component into a liquid stream. More generically this invention relates to mass transfer processes which term is understood to refer to mass and/or heat transfer between gas and liquid phases.
The processes to which this invention specifically relates employ dual-flow fractionation trays in which a liquid passes through perforations in the tray deck in the downward direction while vapor passes upwardly through the same perforations. Such bi-directional flow is avoided in conventional mass transfer trays by the provision of devices to inhibit downward flow of liquid through the perforations.
In processes that use dual flow devices, a tower is provided with a plurality of fractionation trays arranged generally horizontally within the tower. In the process a gas or vapor is introduced at the base of the tower and passes upwards through the perforations in the decks of the fractionation trays. Meanwhile a liquid is introduced at the top of the tower and percolates downward passing through the same perforations in the fractionation trays to the tray below.
Dual flow trays are not provided with downcomers to conduct liquid and vapor/liquid foams down to the next lower level as is the case with conventional valved-perforation, (or "cross-flow"), trays. In such trays a horizontal flow direction across the surface of the tray is established by the location of the downcomer and this aids in producing efficient vapor/liquid contact. Cross-flow trays are therefore quite different in operation and essential design criteria.
The key to efficient operation of a dual flow process is the balancing of the tray pressure drop such that the upward-flowing gas/vapor does not overwhelm the downward-flowing liquid thereby providing efficient contact between liquid and vapor. At the same time care must be taken to ensure that downflowing liquid does not establish a stream which would pass directly down through the perforations to the tray below, thus by-passing the gas/vapor.
In a conventional dual flow process the balance is sought by controlling the size and number of perforations in the tray deck. However this is not an approach that promotes good contact between vapor and liquid, especially if the vapor flow is near the minimum extreme of the permitted designed pressure range. It also limits the range of flow conditions over which the process can function efficiently.
It has now been found that dual flow processes can be designed that permit efficient operation over a wide range of vapor rates while offering minimal added resistance to flow in either direction.
The present invention provides a dual flow mass transfer process using a simple gas/liquid perforated tray device that can easily be installed and which provides a highly effective means of contacting liquid flowing down through perforations in the tray with gas flowing up through the perforations.